The present invention relates generally to a control apparatus for controlling a system of an automotive vehicle in response to sensed dynamic behavior and in response to various road and driving conditions, and more specifically, to a method and apparatus for controlling the yawing and lateral motion of the vehicle by determining its side slip angle.
Vehicle dynamics control systems for automotive vehicles have recently begun to be offered on various products. Vehicle dynamics control systems typically control the yaw of a vehicle by controlling the braking effort at the various wheels of the vehicle, which is also called yaw stability control (short to YSC) or ESP. YSC systems typically compare the desired direction of the vehicle based upon the steering wheel angle and yaw rate sensor and the direction of travel, and the vehicle side slip angle and the desired side slip angle. By regulating the amount of braking at each corner of the vehicle, the desired direction of travel may be maintained, and the severe lateral sliding of the vehicle may be avoided. Typically, YSC systems do not address the influence vehicle body attitude variations on the vehicle side slip angle, one of the key variables needed to be regulated. For high profile vehicles in particular, the activation of the yaw stability control system is usually coupled with the introduction of vehicle attitude variations. On the other hand, the road condition also contributes to vehicle global attitude variations. For example, the driver""s aggressive steering input could cause large roll attitude variation of the vehicle; during off-road driving, the large road bank and slope will be experienced through vehicle attitude changes. Neglecting the effect of the vehicle attitude information might cause control problems during aggressive maneuvers or maneuvers on banked road. For example, a vehicle with large lateral acceleration maneuvers could achieve 6 degree of relative roll angle between the vehicle body and a level road surface. If such a vehicle is driven in 45 mph, an error of rate 3 deg/sec will be introduced in many known side slip angle computation methods. If the same vehicle is driven on a 10 degree banked road, an error of rate 8 deg/sec will be introduced. That is, a 2 second maneuver in this case will end up with 16 degree error in side slip angle estimation.
On top of the yaw stability control system, the vehicle may also be equipped with the roll stability control system (short to RSC). The roll stability control system aims to alter the vehicle attitude such that its motion along the roll direction is prevented from achieving a predetermined limit (rollover limit) with the aid of the actuation from the available active systems such as controllable brake system, steering system and suspension system. The vehicle body attitudes are one type of key variables estimated in the roll stability control systems. It is desired to use such vehicle attitude information to help get accurate vehicle side slip angle for YSC system.
The side slip angle xcex2 and its velocity (time derivative) measure the lateral stability of a driving condition of a moving vehicle. It has been used in vehicle yaw stability control systems, for example see U.S. Pat. No. 5,732,377. There are two main methods used in the literature. The first method solves the side slip angle xcex2 from a nonlinear differential equation, which is obtained approximately as the following:                               β          .                =                                                            a                y                            -                                                ω                  z                                ⁢                                  v                  x                                                                    v              x                                -                                                                      v                  .                                x                                            v                x                                      ⁢            β                                              (        1        )            
where xcex1y is the lateral acceleration, xcfx89z is the yaw rate, and xcexdx is the scalar velocity of the vehicle. The second method uses the so-called linear bicycle model to characterize the side slip angle xcex2, then solve it based on the available sensor signals. Examples are found in U.S. Pat. Nos. 5,732,377 and 6,128,569 where the steering wheel angle sensor signal is usually used.
In U.S. Pat. No. 5,732,377 it is argued that the disadvantage of the first method is the sensitivity to measurement noise and the cumulative integration error (or the drift of the integration), which could make the determination of the side slip angle xcex2 highly inaccurate. However, the first method is valid even when the vehicle is driven in the nonlinear range of the vehicle lateral dynamics. This, however, is the operating range which benefits from yaw stability control (YSC). Although the second method is not as sensitive to sensor noise and does not have integration drift problem, it is only valid when the vehicle is driven in the linear range of its lateral dynamics. As a result, this method yields a xe2x80x9clinearxe2x80x9d xcex2 which a yaw stability control system should not respond to and which is far from the dynamic range that the YSC system is intended to control.
In order to take advantage of both methods, U.S. Pat. No. 5,732,377 uses the combination of the side slip angle from the first method and the second to calculate the actual xcex2 by using the weighted sum. This way of determining side slip angle still has the following drawbacks: (i) the cumulative integration error is not eliminated, the weighted sum only reduces the impact of this error source in comparison with each individual method; (ii) the estimation from the second method is not accurate at all when the vehicle is driven in the nonlinear range of its lateral dynamics; (iii) the cornering stiffness used in the second method is directly related to the road surface friction xcexc, a constant value would not be able to cover road conditions from low xcexc surface to high xcexc surface. Besides those disadvantages, the first method is approximately obtained by assuming 2xcex2 as a small angle instead of xcex2, i.e., the dynamic range for xcex2 is only half of what is intended in those known methods. Another drawback of this system is that the vehicle attitude influence in the determination of xcex2 is not considered.
It has been observed that whenever a vehicle turns, the vehicle experiences certain attitude changes, and the activation of YSC system sometimes coupled with even large vehicle attitude changes. On the other hand, prior to entering a rollover event, the vehicle will experience dramatic attitude changes. A large roll attitude change may introduce a large gravity effect in the output of the lateral acceleration sensor. Hence during aggressive maneuvers, the first method is no longer accurate due to the large gravity contamination in the lateral acceleration sensor signal.
It would therefore be desirable to provide a method that improves the side slip angle determination used in many existing YSC systems by compensating the gravity effect through vehicle attitude, by considering its true physical dynamic region and by dynamically blending.
The present invention provides an improved way to determine the side slip angle of a motor vehicle driven on all road and driving condition. A yaw stability control system or other safety systems (like roll stability control system, etc) uses the improved side slip angle to command a handling and/or safety device.
In one aspect of the invention, a method and system for determining a side slip angle for an automotive vehicle includes various sensors such as a yaw rate sensor, a speed sensor, a lateral acceleration sensor, a roll rate sensor, a steering angle sensor, and a longitudinal acceleration sensor. Each of the sensors are coupled to a controller that determines a side slip angle velocity in response to the sensor signals. The side slip angle velocity is compensated for due to gravity and vehicle attitude changes. Also, the side slip angle velocity is compensated for due to the non-linearity of the side slip angle. The side slip angle velocity is integrated, preferably with an anti-drift integration filter (to determine an integrated side slip angle). A steady state side slip angle is also determined based on the sensors such as the yaw rate sensor and the lateral acceleration sensor. The steady state side slip angle is filtered using a steady state recovery filter. The integrated side slip angle and the steady state side slip angle are combined dynamically (in comparison with the weighted sum of constant weights) to form a side slip angle estimate.
Advantageously, drift due to integration errors are removed from the system. Also, the system allows the side slip angle to be in a relatively large dynamic range in contrast to previously known systems.
Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.